Two of the main operations performed by analogue circuitry in a radio application are filtering and analogue to digital (A/D) conversion. A/D conversion enables a received analogue signal to be digitised so that digital signal processing techniques can be performed on it. Accordingly, an A/D converter (ADC) must be carefully designed so that the digitised signal is a sufficiently faithful approximation to the analogue signal if high quality performance is to be achieved.
The two basic elements required in an ADC are a sampler for sampling an analogue input signal, and a mechanism for comparing the sampled signal with quantized voltage levels and selecting the closest matching level as the digital output voltage.
The criteria which are taken into account in the design stage are the sampling rate, which determines the amount of the input signal to which the conversion process can be applied, and the resolution of the converter, which determines the number of digital voltage levels which can be used to represent the input signal. Clearly the higher the sampling rate and the higher the resolution, the more faithful the conversion of the signal will be. The required sampling rate will be related to the A/D conversion bandwidth which is needed for a particular application.